This invention relates to conversion of linear alkanes, such as n-butane, and more particularly relates to conversion of n-butane to higher value hydrocarbons such as butylenes, isobutane and aromatics using a catalyst composition comprising a platinum metal component and a zincosilicate component.
In many instances it is desirable to convert an alkane such as linear alkane or a molecule containing a linear alkane segment into an alkene by dehydrogenation, a branched molecule by structural isomerization, or an aromatic species. Such alkenes and branched molecules then can be reacted further such as by polymerization or oxidation to form useful products. Normal butane is a linear alkane containing four carbon atoms which is obtained commercially by separation from natural gas and as a petroleum refinery by-product. Further, as government regulations mandate a reduction in the reid vapor pressure (RVP) of gasoline, an excess of light gases such as butane arises.
Other reasons for an increased supply of light paraffins such as n-butane include the higher severity operation of the reforming process in order to maintain a high octane rating in the absence of or reduction of the lead content in gasoline, the increased use of oxygenates such as methyl tertiary butyl ether (MTBE) and ethanol resulting in the removal of butanes from the gasoline pool, the increased demand for jet fuel necessitating increased gas oil hydrocracking resulting in more light gas production, and the increase in operating temperatures in fluidized catalytic crackers also resulting in more light gas production. Thus, there is a great incentive to investigate means for converting these materials into more valuable liquids such as transportation fuels or chemical feedstocks. As such, n-butane is a relatively inexpensive feedstock.
N-butylenes and isobutylene are both useful in isobutane alkylation to produce high-octane isoparaffinic gasoline. Isobutylene is a branched four-carbon olefin monomer useful in the manufacture of polyisobutylenes which can have various properties depending on the manner of polymerization. For example, both crystalline polyisobutylene and viscous polyisobutylene can be manufactured according to well-known processes in the art. In addition, isobutylene is used in the manufacture of methyl-t-butyl ether which is useful as an octane booster in gasoline. Conventionally, butylenes, including isobutylene, are obtained as a by-product from refinery processes such as catalytic or thermal cracking units.
The prior art teaches various dehydrogenation processes employing catalysts containing a platinum component and/or a zinc-containing component wherein the zinc component is either in an amorphous phase or associated with a crystalline molecular sieve.
U.S. Pat. No. 4,260,839 (Chen et al.) discloses a process for converting ethane to C.sub.3 +hydrocarbons using a catalyst comprising a ZSM-5 type aluminosilicate containing a minor amount of added zinc in combination with a Group VIII noble metal or Group IB metal wherein the metals have been incorporated into the catalyst in any convenient process such as impregnation, deposition, or ion-exchange.
U.S. Pat. No. 3,875,253 (Huang) discloses a process for dehydrogenating normal paraffins using a catalyst comprising cobalt, zinc or mixtures or oxides thereof and one or more noble metals of the platinum or palladium families deposited on a low acidity alumina.
U.S. Pat. No. 3,941,871 (Dwyer et al.) discloses crystalline metal organosilicates that are free of aluminum and/or gallium and possess an X-ray diffraction pattern similar to that of ZSM-5 zeolites. Example 10 of the subject patent describes a crystalline organosilicate containing both zinc and sodium. The patent further discloses that the family of crystalline metal organosilicates can contain a hydrogenation component such as platinum and be used for hydroisomerization of normal paraffins and olefin isomerization.
U.S. Pat. No. 3,539,651 (Hepp et al.) discloses a catalytic dehydrogenation process employing a catalyst that contains platinum and zinc aluminate.
U.S. Pat. No. 3,600,332 (Hunter et al.) discloses an alkane dehydrogenation catalyst containing platinum group metal ions exchanged onto an aluminosilicate zeolite, a pore size less than about 5 .ANG..
U.S. Pat. No. 3,755,198 (Stratenus) discloses a catalyst-containing zinc added to a supported noble metal suitable for use in the dehydrogenation of paraffins to monoolefins.
Another dehydrogenation catalyst is disclosed in U.S. Pat. No. 3,790,504 (Duhaut et al.) which catalyst contains platinum, iridium, and zinc or a zinc compound supported on a carrier such as alumina. U.S. Pat. No. 3,880,776 (Box, Jr. et al.) discloses a dehydrogenation catalyst containing a Group VIII metal supported on a zinc aluminate spinel.
European Patent Application No. 0 124 998 discloses a crystalline zincosilicate having a structure strongly resembling a Theta-1 aluminosilicate. The zincosilicate may also be loaded with metals via ion-exchange, mixing and/or impregnation, these metals being selected from the group consisting of Groups IB, IIIB, IIIA, IVA, VA, VB, VIIB and VIII. The specific metals listed include copper, silver, zinc, alumina, gallium, indium, thallium, lead, antimony, bismuth, chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium, platinum, rhenium, and the rare earths. The zincosilicate-containing catalysts can be used for any of the following reactions: alkylation, dealkylation, transalkylation of aromatics, dehydrocyclodimerization, aromatization, isomerization, dehydrogenation, hydrogenation, cracking, hydrocracking, cyclization, oligomerization, polymerization, etherification, and dehydration.
In a paper entitled "Effective Conversion of Paraffins to Aromatics on Pt Ion-Exchanged Ga- and Zn-Silicates," T. Inui et al., Studies of Surf. Sci. Catal., No. 37, Innovation in Zeolite Materials Science 1988, pp. 487 through 494, a platinum-Zn-silicate catalyst is disclosed that effects the conversion of paraffins to aromatics. The paper states that all of the X-ray diffraction patterns of Zn-silicates with different Si/Zn ratios were similar to that of H-ZSM-5, indicating that the Zn-silicate has a pentasil structure. The subject sieves were prepared by using the rapid crystallization method wherein the authors maintain that zinc is incorporated in the crystal structure. The authors further maintain that the selective conversion of light paraffins to aromatics is effected first by the dehydrogenation of paraffin to olefin accelerated by platinum. These olefins are then selectively converted to aromatics with the assistance of zinc. The selectivity to aromatics is reported to be about 47 wt.%.
Another paper entitled "Zinc and Aluminum Substitutions in MFI-Structures; Synthesis, Characterization and Catalysis," W. J. Ball et al. Studies Surf. Sci. Catal., No. 28, New Dev. Zeolite Science Technology (1986), pp. 951-956, discloses a catalyst that contains zinc substituted into the MFI framework of a molecular sieve. The subject MFI zincosilicates were used to crack n-hexane. The paper observed that nonframework zinc species dehydrogenate n-hexane thereby reducing the activation energy for cracking over zincosilicates.
The traditional alkane dehydrogenation processes have several drawbacks. The UOP Oleflex .TM. process as disclosed in, "Oleflex: C.sub.2 -C.sub.5 Dehydrogenation Updated," B. V. Vora et al., Energy Progress, Vol. 6, No. 3, 1986, pp. 171 through 176, employs a platinum-containing catalyst and requires a hydrogen recycle stream to maintain catalyst stability. The chromia/alumina catalyst employed in the Catofin .TM. process as disclosed in, "Catalytic Dehydrogenation of Liquefied Petroleum Gas by the Houdry Catofin .TM. and Catadiene .TM. Processes," R. G. Craig et al., R. A. Meyers (Ed.), Handbook of Petroleum Refining Process, McGraw-Hill, 1986, pp. 4-3 through 4-21 requires frequent regeneration.
Dehydrogenation reactions are fast and reversible. They are limited by thermodynamic equilibria constraints. Low pressures and high temperatures shift the equilibria favorably toward dehydrogenated products. However, conventional platinum- and chromia-containing catalysts suffer rapid deactivation by coking under these severe conditions.
Thus, there is a need for an improved light paraffin dehydrogenation process wherein the catalyst system maintains stability, preferably without hydrogen circulation.
There are several advantages afforded by the decreased use or the lack of use of a hydrogen diluent. Specifically, the dehydrogenation process becomes more economically attractive as the amount of hydrogen used is decreased. Further, as mentioned above, if the partial pressure of hydrogen is reduced or eliminated, the yield of dehydrogenated products is increased because the thermodynamic equilibrium constraints of the dehydrogenation process favor product formation at lower reaction pressures.
Accordingly, the present invention provides for an improved dehydrogenation process wherein the catalyst possesses a high degree of stability with or without a hydrogen diluent in the feedstream, and possesses a high selectivity towards olefins.